1. Field of the Invention
The present invention is directed generally to systems for monitoring the clearance between fixed and rotating parts within a combustion turbine and, more particularly, to systems of that type that operate on line.
2. Related Art
The clearance between the stationery seals of a turbine and compressor and the tips of the blades must not be so great to permit an excessive amount of air in the case of the compressors and combustion gases in case of the turbines to pass between them and thereby reduce the efficiency of the turbine. On the other hand, clearances can not be too small because high centripetal loading and high temperatures may cause blades to lift or grow radially. Such blade lifting or radial growth can cause blade tips to rub the stationery seal and may eventually cause seal, and blade tip damage.
In addition, the differences in thermal response time of the various combustion turbine components can result in the mechanical interference between stationery and moving parts under certain conditions. This is certainly the case during the restart of a hot turbine where contact between the compressor/turbine blading and the blade ring has resulted in massive compressor and turbine damage. Even a slight rub will destroy blade seals and reduce the efficiency of a combustion turbine. The obvious solution is to prolong restart until the turbine cools. This requires many hours. However, the situation is further complicated by the competing need to spin-cool the turbine following shutdown to prevent sagging or humping of the rotor. Both can only be done if the blade clearance is accurately measured, and appropriate action is taken based on this on-line measurement.
Capacitance blade clearance probes are used to study blade clearance patterns to establish restart and spin-cool rules. These sensors have proven both inaccurate and unreliable as an engineering tool and are thus even less suitable for commercial on-line monitoring.
A number of blade clearance systems have been developed for steam turbines, such as those described in U.S. Pat. No. 4,987,555. These systems depend upon indicia on the blades shroud to obtain a meaningful proximity measurement. However, the approaches do not appear readily applicable to combustion turbine applications.
Accordingly, a need exists for an on-line combustion turbine blade clearance monitor that can accurately measure, in real time, the clearance between the blades and blade ring of a combustion turbine.
An on-line, real time blade clearance monitor is provided to meet the foregoing objectives. The clearance monitor includes an insertion probe that is positioned within a stationery member portion of the turbine and reciprocally moveable within a cavity, preferably radially in line with the turbine blade at the point where the blade is closest to the stationery member. A proximity sensor is supported at one end of the insertion probe closest to the turbine blade and a connecting rod is affixed to the other end of the insertion probe. The connecting rod is reciprocally driven by a motor such as a stepper motor or a pulsed D.C. motor and resolver that provides a positional output to a computer. A calibration indicia is provided that results in a unique output of the proximity sensor when the proximity sensor is positioned flush with an interior surface of the stationery turbine member that faces the turbine blade.
In operation, the insertion probe starts from a point where the proximity sensor is flush with the interior surface of the stationary turbine member. The insertion probe is then advanced towards the blade until the proximity sensor output is a preselected distance from the turbine blade as represented by a given output of the sensor and recognized by a computer controller. The computer controller also monitors the motor drive to determine the distance the insertion probe has been moved towards the compressor blade. The computer controller then calculates the clearance distance between the blade and the stationery turbine member by adding the preselected distance to the monitored advancement of the insertion probe as indicated by the motor drive to the computer controller.
Preferably, the insertion probe is constructed of materials that will not substantially damage the blade should the insertion probe and the blade come in contact. Desirably, the electrical continuity of the proximity sensor is monitored to determine whether any such contact has occurred. If the sensor is disabled, the computer controller directs the motor to withdraw the insertion probe to the calibration point.
During turbine operation or cool-down while rotation of the rotor is maintained, the foregoing clearance measurement is monitored at periodic intervals such as every five minutes, though the steps of the method of this invention may be repeated as often as every five seconds.